1. Field of the Invention
The present invention is an innovative apparatus, system and method for spacecraft navigation employing the use of extrasolar planetary system motion. Spacecraft navigation can generally be described as, but not limited to, the determination of a spacecraft's position, velocity and attitude at certain times as well as the determination of orbital parameters and trajectories. Extrasolar planetary systems are star systems other than the Sun that have planetary companions. The present invention relates to several different fields including spacecraft hardware, software, navigation, astronomy, Doppler spectroscopy methods and astrometric techniques.
2. Description of the Related Art
Precise determination of spacecraft position and velocity is necessary in order to achieve mission success for operations of near Earth and interplanetary missions. Onboard flight technologies can provide spacecraft position, navigation and timing (PNT). Areas of related art include traditional spacecraft navigation hardware and software, tracking such as NASA's Deep Space Network (DSN), the Global Positioning System (GPS), X-ray navigation and extrasolar planetary detection.
Space navigation traditionally relies on initial spacecraft position, velocity and attitude estimates that are regularly updated by onboard inertial measurement unit (IMU) data. An IMU is a device that measures a spacecraft's velocity changes and orientation using a combination of accelerometers and gyroscopes. Spacecraft orientation can also be aided by a star tracker, which is an optical device that measures the relative position(s) of star(s) against the celestial background using photocells or a charged couple device (CCD) camera. Additional components such as horizon or sun sensors are also traditionally employed.
Methods of onboard orbit and position determination involve accurate updates to the spacecraft's navigation state matrix (“Nay State”). Periodic updates from external signals can be processed by onboard software algorithms and filters. As an example, in low Earth orbit (LEO), the Nay State can be refined by employing Kalman filtering and data from terrestrial navigation aids such as C band radar tracking or the GPS. There are various ways to implement these software filtering capabilities, one of which is NASA's GPS Enhanced Onboard Navigation Software (GEONS).
GEONS supports the acceptance of many one way forward Doppler, optical sensor observation and accelerometer data types. GEONS was designed for autonomous operation within the limited resources of an onboard computer. It employs an extended Kalman filter (EKF) augmented with physically representative models for gravity, atmospheric drag, solar radiation pressure, clock bias and drift to provide accurate state estimation and a realistic state error covariance. GEONS incorporates the information from all past measurements, carefully balanced with its knowledge of the physical models governing these measurements, to produce an optimal estimate of a spacecraft's orbit. GEONS' high-fidelity state dynamics model reduces sensitivity to measurement errors and provides high-accuracy velocity estimates, permitting accurate state prediction.
Interplanetary missions typically employ tracking services from NASA's DSN, which provides radiometric ranging, Doppler and plane-of-sky angle measurements. For spacecraft ranging, a signal is sent from one of the DSN stations on Earth to the spacecraft, which in turn sends a signal back to Earth. The round trip transit time is measured to determine the line of sight slant range. Two-way Doppler tracking also uses a signal sent to and from a spacecraft; by looking at the small changes in frequency, the spacecraft velocity along the line of sight can be determined.
In general, angular measurements can be made using multiple DSN ground stations that receive spacecraft transmissions simultaneously during overlapping viewing periods. An additional method used by DSN is delta differential one-way range (ADOR). This is a Very Large Baseline Interferometry (VLBI) technique that uses two ground stations to simultaneously view a spacecraft and then a known radio source (such as a quasar) to provide an angular position determination.
Unfortunately, DSN resources are limited and its accuracies degrade over large distances. Onboard spacecraft navigation systems that can reduce tracking requirements for the DSN are currently needed. Furthermore, GPS satellites orbiting the Earth are of limited use for deep space missions. Thus, hardware and software systems and methods that provide precise navigation solutions using a methodology that is independent of Earth based systems are not only innovative and novel but are currently needed for spacecraft navigation.
Some recent research and development with autonomous deep space navigation has examined the use of pulsed X-ray radiation emitted by pulsars. Such investigations designate X-ray millisecond pulsars as a potential signal source to be observed by a spacecraft. However, the specific characteristics of pulsars are limiting and very different from main sequence stars such as our sun. The current invention uses the properties of main sequence stars and their associated extrasolar planets.
In the past 15 years or so, over 700 extrasolar planets (or exoplanets) have been discovered orbiting around 560 main sequence stars (some stars have multiple detected exoplanets). These stars are evenly distributed throughout the celestial sphere and most are within several hundred light years (ly) of Earth. Some potential exoplanet reference stars include, but are not limited to, Epsilon Eridani (10 ly away), Gliese 86 (36 ly), 47 Ursae Majoris (43 ly), 55 Cancri (44 ly), Upsilon Andromedae (44 ly), 51 Pegasi (48 ly) and Tau Bootis (49 ly). All have well known characteristics and are even visible to the naked eye.
Before the discovery of exoplanets, the only planets known to exist were those in our own solar system. The motion of the Earth about our Sun is well understood and the whole solar system in fact rotates around a common center of mass, known as the barycenter. Astronomers, in order to detect possible planets around stars other than our Sun, had to separate known and unknown stellar motion to determine the motion of other stars about their own barycenters. The initial theory postulated that if exoplanets did exist, their orbits would cause their parent star to wobble by a small amount. This motion was indeed detected, yielding numerous exoplanet discoveries. The measurements to date have produced now well known patterns of highly stable, predictable exoplanetary system stellar motion with respect to our own solar barycenter. This exoplanetary system stellar motion can be used to determine the location of a spacecraft both within and outside of our solar system. This is the methodology employed by the present invention.